Bar evolution in edge-on galaxies: A demographic study of boxy/peanut bulges

¹ Indian Institute of Science Education and Research Bhopal 462066, India
² Universidad Andrés Bello, Facultad de Ciencias Exactas, Departamento de Física y Astronomía, Instituto de Astrofísica Fernández Concha 700 Las Condes Santiago RM, Chile
³ Indian Institute of Astrophysics Bangalore 560034, India

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 May 2025
Accepted: 17 November 2025

Abstract

Context. Boxy/peanut and X-shaped (BP/X) bulges are prominent features in edge-on disk galaxies and they are believed to represent vertically thickened bars. Despite their relevance in bar evolution, a statistically robust census of these structures in large surveys remains lacking.

Aims. We aim to provide the largest catalog of BP/X structures in edge-on galaxies to date, along with an investigation of their properties and role in shaping galaxy scaling relations.

Methods. We selected a sample of 6684 edge-on galaxies from SDSS DR8 using Galaxy Zoo classifications, requiring a high edge-on probability (> 0.9) and a minimum of ten independent votes. We performed a two-dimensional (2D) image decomposition using GALFIT to obtain the structural parameters. The residual images were visually inspected to classify BP/X features into four categories: strong both-sided, both-sided, one-sided, and control (no BP/X). We also estimated the stellar mass, distance, and physical size for each galaxy.

Results. Out of 6653 classified galaxies, we identified 1673 (∼25%) with both-sided BP/X features, 504 (∼8%) strong, and 1169 (∼17%) weak, as well as 1112 (∼17%) one-sided structures, making up a total of 2785 BP/X-hosting galaxies (∼42%). We find that one-sided structures, likely signatures of ongoing buckling, are more frequent than strong both-sided bulges across all stellar masses. The fraction of BP/X bulges increases with stellar surface mass density, indicating a connection with bar formation in dense discs. We also find that galaxies with strong BP/X bulges contribute to increased scatter in the stellar-mass-size and stellar-mass-surface-density relations, particularly at higher masses.